Agroforestry buffers on nitrogen reduction in groundwater on a grazed hillslope

Abstract

Agricultural practices often contribute to the transport of solutes into groundwater; thus, low‐cost strategies that extract nutrients from groundwater are essential to address water pollution. This study evaluated the effects of agroforestry (tree + grass; AB [cottonwood {Populus deltoides Bortr. ex Marsh.}]) and grass buffers (GB; [tall fescue Schedonorus phoenix (Scop.) Holub, Red clover {Trifolium pretense L.}, and Lespedeza {Lespedeza Michx}]) on groundwater nitrogen (N) concentrations. The experiment consisted of two grazing watersheds, one with an AB and another with a GB treatment. Buffers were not grazed since 2001. Three wells representing summit, backslope, and foot‐slope positions were installed at each watershed. Water samples were collected biweekly from November 2019 to January 2022 and analyzed for total nitrogen (TN) and dissolved N (DN). Dissolved nitrogen (DN) and TN concentrations after the AB in the foot‐slope well were 99% (5.36–0.06 mg L−1) and 85% (9.04–1.37 mg L−1) lower than the mean concentration of the summit and backslope wells. Similarly, DN and TN concentrations after the GB in the foot‐slope well were 94% (1.95–0.11 mg L−1) and 62% (2.86–1.07 mg L−1), lower than the mean concentration of the summit and backslope wells. Dissolved N concentrations were lower during warm periods probably due to plant uptake and denitrification in the buffer zone. Results showed that buffers, especially with deep‐rooted trees in the proximity of the water table, decreased TN and DN concentrations in groundwater and can be used as a conservation practice to address water pollution.